How to share semaphores between processes using shared memory

Question

I have to synchronize N client processes with one server. These processes are forked by a main function in which I declared 3 semaphores. I decided to use POSIX semaphores but I don't know how to share them between these processes. I thought that shared memory should work correctly, but I have some questions:

• How can I allocate the right space of memory in my segment?
• Can I use sizeof(sem_t) in size_t field of shmget in order to allocate exactly the space I need?
• Does anyone have some examples similar to this situation?

Answer 1

It's easy to share named POSIX semaphores

• Choose a name for your semaphore

  #define SNAME "/mysem"
  

• Use sem_open with O_CREAT in the process that creates them

  sem_t *sem = sem_open(SNAME, O_CREAT, 0644, 3); /* Initial value is 3. */
  

• Open semaphores in the other processes

  sem_t *sem = sem_open(SEM_NAME, 0); /* Open a preexisting semaphore. */
  

If you insist on using shared memory, it's certainly possible.

int fd = shm_open("shmname", O_CREAT, O_RDWR);
ftruncate(fd, sizeof(sem_t));
sem_t *sem = mmap(NULL, sizeof(sem_t), PROT_READ | PROT_WRITE,
    MAP_SHARED, fd, 0);

sem_init(sem, 1, 1);

I haven't tested the above so it could be completely bonkers.